At 33, Roger Federer has been in age-defying form this year and he arrived as the oldest player to qualify for the year-end tournament at London in his record 13th consecutive appearance. Federer beat Raonic 6-1, 7-6 on Monday in his opening match.

Roger Federer was back in the groove at the ATP World Tour Finals as the six-time champion defeated Milos Raonic 6-1, 7-6 (7/0), while Japan's Kei Nishikori made a dream debut with a surprise 6-4, 6-4 victory over Andy Murray on Sunday.

Federer last won the prestigious season-ending event at London's O2 Arena in 2011 and the world number two looks in the mood to add another title to his collection on the evidence of a powerful performance, featuring 24 winners, that avenged his loss to Raonic in the Paris Masters quarter-finals last month.

Raonic, 23, is making his Tour Finals debut as the first player born in the 1990s to compete in the event after a breakthrough year that included a run to the Wimbledon semi-finals.

But at 33, Federer has been in age-defying form this year and he arrived as the oldest player to qualify for the tournament in his record 13th consecutive appearance.

Federer is set to become the oldest player to finish a season in the world's top two and still has a slim chance of becoming the oldest player to end the season as world number one if he overhauls Novak Djokovic in the race for top spot.

The 17-time Grand Slam winner brushed aside Raonic in straight sets in the Wimbledon semi-finals earlier this year and he wasted little time reestablished his supremacy in their latest London meeting.

Raonic has fired down over 1,000 aces this year and he looked to overpower the Swiss by firing down 138mph and 141mph bombs in his opening service game.

But Federer wasn't intimidated by that barrage, returning both massive serves to earn three break points that swiftly turned into a 2-0 lead.

If that was disheartening for Raonic there was worse to come as Federer, defusing the Canadian's serve with ease, broke again in the sixth game before closing out the set in emphatic fashion.

Despite a chastening first set, Raonic showed commendable resilience and began to assert himself at the start of the second, forcing Federer to dig deep to save two break points in the fourth game.

Raonic's serve was in the groove now and, with his groundstrokes equally vigorous, the Canadian earned a set point at 6-5.

Yet Federer managed to escape that predicament and forced a tie-break, which he took in ruthless fashion to seal the win.

"I was very happy with how I performed. The second set was much tougher. I don't think he played a great breaker but it was a great one to win," Federer said.

"It's a small relief. We have a tough group here so it's always going to be hard advancing but it brings me a step closer."

- Nishikori on the rise -

Nishikori is the first Asian singles player to qualify for the Tour Finals and he rose to the occasion with his first ever victory over former Wimbledon champion Murray at the fourth attempt.

The 24-year-old had failed to win even a single set in those losses to Murray, but the last came over two years ago and he has improved greatly since then.

He took the game to Murray right from the start and hit 20 winners in a 95-minute triumph that gives him a good chance of qualifying for the semi-finals.

"Maybe in the beginning I was a little bit tight, but I started feeling better and the second set was almost perfect," Nishikori said.

"I knew I had to be more aggressive than usual and that's how I won. It's my goal to go to the semi-final and final."